Fix QuaternionAlgebra bug that caused QuaternionAlgebra( [1], 2, 5 ).1 to error the second time it was invoked

[fingolfin]

Also ensure that the exact same code is used to produce the
resulting algebra regardless of whether it is created freshly
or from the cache. This revealed a bug: A.1 only worked
in the former case, not the latter, which could lead to subtly
broken code. Before this patch, the following happened:

gap> A:= QuaternionAlgebra( [1], 2, 5 );
<algebra-with-one of dimension 4 over Rationals>
gap> A.1;
e
gap> B:= QuaternionAlgebra( [1], 2, 5 );
<algebra-with-one of dimension 4 over Rationals>
gap> B.1;
Error, illegal access to record component `obj.1'

This is now fixed.

Resolves #2940

[ThomasBreuer]

Very nice, the caching technicalities are delegated to GET_FROM_SORTED_CACHE.
The test failures are due to a subtle change of the behaviour.
After the changes, each call of QuaternionAlgebra creates a new algebra, and the \. access to generators is not supported since the filter IsFullSCAlgebra is not set.
The situation was much worse with the old code: The first call for each coefficients family simply took the newly created algebra, in which the filter was set, whereas subsequent calls used the cache and then created a new algebra, without the filter.
Thus it depended on the GAP session whether \. access to generators worked.
I regard the changes as a bugfix, perhaps even an entry for the release notes would be appropriate.

[ThomasBreuer]

Here a new algebra gets created; setting the filter IsFullSCAlgebra would be useful.

[fingolfin]

Great catch, fixed now, and commit message adjusted. I agree this is a bugfix.

[ThomasBreuer]

Very nice.